The extravasation of plasma proteins (albumin, IgG, and complement C3) into the cerebral cortex was studied in streptozotocin-induced diabetic rats using im-munohistochemical techniques. The results indicate that albumin, but not IgG or complement C3, selectively enters the cerebral cortex within 2 wk after induction of diabetes. It is suggested that albumin may be an oncotic substance that contributes to diabetic cerebral microangiopathy, astrocytic swelling, and the cerebral edema that occasionally is seen during fluid and insulin therapy of juvenile ketoacidotic diabetes.